1. Field of the Invention
This invention pertains to the field of security devices. More particularly, this invention pertains to security systems for motor vehicles that can distinguish between attempted break-ins and the sound from thunder, voices and the shock from passing vehicles to provide a more reliable indication of an existing security breach.
2. Description of the Prior Art
Virtually all breaches into secured areas, such as to a home, office or automobile, involve an intrusion or invasion into the protected area. The burglar or vandal physically enters the secured area to either damage the interior or remove some valuables or, in the case of automobiles, to literally steal them. The prior art has developed numerous security systems for detecting these intrusions. Many of the more sophisticated systems employ a device known as a "shock sensor" that detects the incoming shock or noise accompanying the intrusion and converts it to electrical signals that are processed by a miniature electronic circuit to eventually trigger an alarm.
The prior art of shock sensors is well-developed. One type incorporates a mechanical spring and relay to detect the motion or shock, however it is rather crude in design and has generally been abandoned. Another type incorporates a magnet suspended in or near a coil of wire on a rubber band. In operation the magnet is jiggled under the influence of a shock and vibrates near the coil to generate a voltage therein. Another type of shock sensor uses a piezo element and spring to accomplish the same task. Still another type uses a microphone to listen for the sound of entry, such as the sound of breaking glass, and another type of shock sensor uses a microphone to detect the air pressure change in the interior of the vehicle when the burglar enters the passenger compartment. All of these devices generate an electric pulse that is processed by other electronics to trigger the alarm.
There is, however, a problem encountered with security devices that remains, for the most part, unsolved and that is the false signals issued by the shock sensor when the automobile is subjected to a non-intrusive shock. These non-intrusive shocks can come from a wide variety of sources. For instance, during storms, claps of thunder or peals of thunder impart a shock to the vehicle that will cause most shock sensors to issue a pulse or signal indicating an intrusion has occurred. A gusty wind against the automobile can produce the same false signal. In addition, if the automobile is parked along a busy street, vibration from a passing truck will also appear as an intrusive shock. The same phenomenon occurs when someone shouts near an automobile.
In dealing with this problem, the prior art has attempted to use two microphones, one installed inside the passenger compartment and the other installed outside the compartment. If both microphones pick up the same shock, then they cancel each other and no warning signal is issued. However, this design requires more components than normal, a longer installation time, more testing and more "tuning" of the circuitry and higher-than-normal maintenance. Accordingly, at present, there is no effective shock sensor that can discriminate between an intrusive and a non-intrusive shock without incorporating a very complicated system of components.